Toric lens with axis mislocation latitude

ABSTRACT

A toric lens for axis mislocation correction has optical topography on at least one surface of the lens which induces a depth of field effect on the eye and enables the principle meridians of power on the lens to align with those on the eye of the wearer such that it compensates for mislocation error if the lens mislocates 1 degree or more. The lens is thin enough to allow sufficient oxygen transmission therethrough to provide satisfactory morphology of the eye of a wearer. The optical topography which induces the depth of field effect may comprise either aspheric topography, spline curve functions, diffractive optics using eschelets, ion impingement or bi-refringence optics.

This application is a 371 of PCT/AU92/00418 filed Aug. 7, 1992.

The present invention relates to toric lens technology and moreparticularly relates to a toric lens which employs non-sphericalsurfaces including aspheric topography which produces a depth of fieldeffect which reduces or neutralises axis mislocation of the lens andimproves the astigmatic lens axis stability latitudes.

For some years now soft and/or hard toric lenses have been manufacturedto correct an ocular phenomenon known as astigmatism. In the case ofthis particular ocular problem the eye has more than one correctableplane of focus.

Whereas a simple ocular problem may be of a short or long sightednature, an astigmatic eye can incorporate infinite variations of eithertype of defect or both. This, therefore requires a highly specializeddesign of lens to be fitted in order to correct this visual defect.

The major criteria involved in correcting astigmatism is accuratelyaligning the principle power meridians of the lens with the principlerefractive meridians meridians in the eye. If this is not doneaccurately, a resultant misalignment error occurs (i.e. the incorrectlyaligned lens powers and the refractive powers of the eye combine tocreate totally different powers from which the eye needs to attaincorrect vision) preventing proper correction of astigmatism. Thisaccuracy is determined by how closely the axis of powers in the lensaligns with those of the eye.

As a rule the contact lens Practitioner can refract the astigmatic eyeto within one degree of accuracy. The manufacturer should be able tocontrol his lens accuracy to within 5 degrees. This is generallyaccepted as an industry standard. If however the lens does not positionexactly on the axis when in vive then a misalignment error occurs. Evenwith the 5 degrees manufacturing tolerance, a misalignment error canoccur but will generally be accepted by the patient if the cylindricalcomponent of their astigmatism is relatively low (eg. around -2.00diopters or lower).

If the lens swings 10 degrees or more in vive due to physiologicalforces exerted during blinking, eye turning, retropulsion of the eyeduring blinking or similar, then the patients vision will becompromised.

Within the industry, most manufacturers design their lenses specificallyto overcome this problem. This can sometimes be at a cost to other veryimportant criteria as for example by making the lens very thick in orderto help location, but at a cost of oxygen deprivation to the cornea anda loss of comfort.

It is well known that spherical surfaced optical elements applied to theeye (both spectacle and contact lenses) will induce spherical aberrationeffects on the retina. Spherical aberration is a common opticalphenomenon associated with spherical surfaces. Spherical aberrationoccurs when rays passing through the periphery of the lens are deviatedmore than those passing through the paraxial zone. The prismatic effectof a spherical lens is least in the paraxial zone and increases towardsthe periphery of the lens. It reduces the clarity and luminence ofobjects focussed by optical elements which are of a spherically surfacednature. Most cameras and telescopes etc. utilise aspheric or doubletlens systems to reduce or eliminate this spherical aberration. A doubletconsists of a principal lens and a somewhat weaker lens of differentrefractive index cemented together. The weaker lens must be of oppositepower, and because it too has spherical aberration, it will reduce thepower of the periphery of the principal lens more than the central zone.

Up until recently most contact lenses were manufactured with sphericalsurfaces due to simplicity of manufacture and as that was the onlytechnology available. The resultant aberrations manifested in the eyewere usually less important to the wearer than the improved comfort,cosmesis etc, afforded over the wearing of spectacles. Recently however,aspheric lens forms have been used more frequently to improve visualperformance.

This has been particularly so in the area of bifocal lenses ormultifocal lenses. Some of these lenses have been designed to reduce oreliminate the spherical aberration presented to the eye and thus form amuch smaller circle of least confusion on the retina. This not onlyimproves visual performance (as in contrast sensitivity and low levellight contrast sensitivity) but also can improve depth of field depth offocus. Some other designs simply utilise aspheric/multi-aspheric orsimilar curves to artificially create greater "depths of field" thanafforded to the patient by simply eliminating or reducing sphericalaberration.

These designs create quite large "depth of field" effects by complexnon-spherical optical curvatures and are generally prescribedovercorrected on the plus side to allow the patient to "artificially"accommodate for near to intermediate visual tasks.

This lens design can be manufactured by all the normal methods availableto manufacturers, but will also suit more advanced technologies such asthe use of bi-refringence. It is generally preferred that thistechnology be used on the anterior surface of the lens due to greaterrefractive index difference afforded by the air/lens interface, but thisis not mandatory. A combination of front and back surface topographywith both surfaces being changed from spherical can be utilised. So toocan back surface topography changes be used effectively. This wouldespecially suit diffractive or diffractive/Aspheric optical techniques.This is like the lens creating an Instamatic camera type effect on theeye whereby the wearer can focus cleanly on near objects and far objectswith little compromise of both.

The present invention seeks to overcome the aforesaid problems byproviding a toric lens which reduces or eliminates the misalignmenteffect which occurs when a toric lens mislocates on the astigmatic eyeand which will stabilise in vivo to within a few degrees but which isstill thin enough to afford the cornea enough oxygen for satisfactorymorphology and to provide good comfort and good cosmesis. In the contextof the present invention, a thick toric lens i.e. one which willstabilise in a satisfactory manner would have a prism or wedge peakthickness of 0.30 mm to 0.45 mm, whereas a thin version of the same lenswould be around 0.15 mm to 0.25 mm.

Some designs have come close to the aspheric toric lens however theyhave not achieved axis mislocation correction ability according to thepresent invention. Usually some compromise must prevail In order tosatisfy the primary objectives; that of accurate, consistent axisstability whilst the lens is being worn in a dynamic environment.

The toric lens according to the present invention satisfies most or allof the above criteria but also provides a built in latitude of axislocation by usage of complex aspheric optical topography.

In its broadest form the present invention comprises:

a toric lens for axis mislocation correction having optical topographyon the surfaces of the lens which induces a depth of field or depth offocus effect on the eye and enables theprincipal meridians of power onthe lens to align with those on the eye of the wearer such that itneutralises mislocation error if the lens mislocates 1 degree or moresaid lens being thin enough to allow sufficient oxygen transmissiontherethrough to provide satisfactory morphology of the eye of a wearer.

Preferrably the depth of field effect is produced by aspheric opticaltopography however the same or a similar effect may be produced bySpline Curve Functions, Diffractive Optics (by Eschelets or similar) orIon implantation or similar or Bi-refringence optics.

Spline Curve Function is a method of describing a complex, non sphericalcurve via the use of weigh points or X Y co ordinates. A Spline curve isthe smoothest possible curve between these points.

Diffractive optics are the reverse of refractive optics. Light enteringthe optical element is split (generally by 1/2 wave length) and then recontinued at any given focal length. Miniature prisms or eschelets areused for this purpose.

Ion implantation is the implantation of one medium into another mediumto change its physical characteristics. In an optical sense, implants amedium of a higher or lower refractive index in specific areas in orderto specifically create different focal lengths. This can be achieved viathe use of a particle accelerator or similar.

Bi-refringence is a phenomenon where a medium can have two focal lengthsat the one time. In nature this occurs in crystals (which are naturallybi-refringent). So too is silicone which is why they make good computerchips. Specifically induced stress at opposing axis can inducebi-refringence in a number of elements.

The present invention will now be described in more detail and withreference to preferred but non limiting embodiments.

According to the invention, lenses are produced with an aspheric surfaceor surfaces with the nature and amount of asphericity being dictated bythe depth of field effect required to neutralise the mislocation error.For a given toric lens power which creates a given amount of mislocationerror an aspheric lens with the requisite depth of field effect willneutralise this mislocation.

For example, if an aspheric or multi-aspheric surfaces lens can create adepth of field effect on the eye of approximately plus or minus 1.50spherical diopters and is manufactured in toric form and a sphericaltoric lens with a power of -2.00/-1.50×180 mislocates by 10 degrees fromthe required axis and therefore creates a mislocation error of+0.26/-0.25×130 then the depth of field effect of the aspheric toriclens will neutralize this error. -2.00 is the power in the meridian ofthe lens of 90° to 270° and in the meridian of 0°-180° the power is-3.50. -2.00/-1.50×180 may be written overall as -2.00/-3.50×180. Theeye needs -2.00/-1.50×180 for perfect vision. When a lens with thispower sits exactly on this axis i.e. 0 -180° this power requirement isfulfilled. If the meridians do not line up and an additional power overthe top is needed to correct the eye, that power is calledoverrefraction. The overrefractions are simply resultant powers whichoccur when the lens mislocates and does not correct the eye's power.

If the lens power required was -3.50/-200×180 and the lens mislocated 10degrees the resultant overrefraction would be +0.35/0.64×130 which againwould be compensated for by the depth of field effect.

Even on a higher power lens such as a -5.00/-500×180 which swung 10degrees where the mislocation error was +0.87/-1.74×130 the asphericeffect would compensate for most of this drop in refractive accuracy.

If for example this lens can be fitted for the distance power and canimprove the depth of field effect by a total of 1.5 DS then anyoverrefraction, whether it be a simple crossed cylinder or a morecomplex lens over an under corrected or over corrected refraction, willbe compensated by up to +0.75 in either spherical or cylindricalmeridians for example +0.75/-1.50 (Composite Powers: not meridignal).

This concept therefore will apply to most astigmatic lenses which mayswing off axis by reasonable amounts in the order of 10°-15°. The rulewhich will apply will obviously be that of higher cylinders beingcompensated when small amounts of mislocation occurs and smallercylinders being compensated even when larger amounts of mislocationoccur.

The advantage of this design however is that in most cases any swingingoff axis in toric lenses presents the wearer with some form of visualcompromise. Some manufacturers of stock toric lenses which do not exceed-2.00 D cylinders claim a 10 degree mislocation allowance is acceptableas the vision will only be comprised to a small extent. The design thatis proposed here will negate this practice in that with low cylinders at10 degrees mislocation there will be little or no drop in visualperformance and that this built in visual compensation will extend tomuch larger cylinders and to greater amounts of axis mislocation.

It will therefore, theoretrically, be possible to manufacture stockastigmatic lenses which have a cylinder power to greater than -2.50 Dwhich can mislocate up to 30 degrees with little drop in visualperformance.

The other advantage this design incorporates is that of manufacture.Toric lenses are by virtue of their complexity difficult to manufacture.The leads to a higher scrap rate and an end higher cost to thepractitioner. They are much

I claim:
 1. A toric lens for axis mislocation correction comprising atoric lens having optical topography on at least one surface of the lenswhich induces a depth of field effect on the eye and enables theprinciple meridians of power on the lens to align with those on the eyeof a wearer such that the depth of field effect compensates formislocation error if the lens mislocates 1 degree or more, said lensbeing thin enough to allow sufficient oxygen transmission therethroughto provide satisfactory morphology of the eye of a wearer.
 2. A toriclens according to claim 1 wherein the optical topography which inducesthe depth of field effect comprises any one of aspheric topography,spline curve functions, diffractive optics using eschelets orbi-refringence optics.
 3. A toric lens according to claim 2 wherein thetopography is applied to the front surface of the lens.
 4. A toric lensaccording to claim 1 wherein the optical topography is aspheric opticaltopography which compensates for axis mislocation greater than 10degrees.
 5. A toric lens according to claim 4 wherein the lens allowsfor compensation at a high cylinder power and/or at a low cylinder powerat mislocation within the range of 10°-15°.
 6. A toric lens according toclaim 4 wherein the lens has a cylinder power to greater than -2.50 Dand the lens can correct mislocation up to 30° with little drop invisual performance.
 7. A toric lens according to claim 6 wherein for aspherical toric lens with a power of -2.0/-1.5×180 and which mislocates10 degrees from the required axis and therefore creates a mislocationerror of +0.26/-0.25×130, an aspheric toric lens having a depth of fieldeffect on the eye of approximately plus or minus 1.5 spherical diopterswill compensate this error.
 8. A toric lens according to claim 6 whereinfor a spherical toric lens with a power of -3.5/-200×180 and whichmislocates 10 degrees from the required axis and therefore creates amislocation error of 0.35/0.64×130, an aspheric toric lens having adepth of field effect on the eye of approximately plus or minus 1.5spherical diopters will compensate this error.
 9. A toric lens accordingto claim 6 wherein for a spherical toric lens with a power of-5.00/-500×180 and which mislocates 10 degrees from the required axisand therefore creates a mislocation error of 0.87/-1.74×130, an aspherictoric lens having a depth of field effect on the eye of approximatelyplus or minus 1.5 spherical diopters will compensate this error.
 10. Atoric lens according to claim 7 wherein the lens allows for compensationat a high cylinder power when a small amount of mislocation occurs andat a small cylinder power when a large amount of mislocation occurs. 11.A toric lens according to claim 10 wherein the lens compensates formislocation error where the refractive error due to mislocation of thelens is not equal in two principal meridians of power.
 12. A toric lensaccording to claim 11 wherein the lens thickness falls within the rangeof 0.15 mm to 0.25 mm.
 13. A toric lens according to claim 8 wherein thelens allows for compensation at a high cylinder power when a smallamount of mislocation occurs and at a small cylinder power when a largeamount of mislocation occurs.
 14. A toric lens according to claim 9wherein the lens allows for compensation at a high cylinder power when asmall amount of mislocation occurs and at a small cylinder power when alarge amount of mislocation occurs.
 15. A toric lens according to claim13 wherein the lens compensates for mislocation error where therefractive error due to mislocation of the lens is not equal in twoprincipal meridians of power.
 16. A toric lens according to claim 14wherein the lens compensates for mislocation error where the refractiveerror due to mislocation of the lens is not equal in two principalmeridians of power.
 17. A toric lens according to claim 15 wherein thelens thickness falls within the range of 0.15 mm to 0.25 mm.
 18. A toriclens according to claim 16 wherein the lens thickness falls within therange of 0.15 mm to 0.25 mm.
 19. An astigmatic lens which reduces axismislocation comprising a toric lens having optical topography whichinduces a depth of field effect on the eye and enables the principlemeridians of power on the lens to align with those on the eye of awearer such that the depth of field effect induced by the opticaltopography is about plus or minus 1.5 spherical diopters to reducemislocation error, said lens being thin enough to allow sufficientoxygen transmission therethrough to provide satisfactory morphology ofthe eye of a wearer.
 20. An astigmatic lens as claimed in claim 19wherein said optical topography comprises any one of aspherictopography, spline curve functions, diffractive optics using eschelets,ion implantation, or bi-refringence optics.